The invention relates to a method for the circulation grinding of brittle grinding stock in the nip of an interparticle crushing high-pressure roll press with rolls, of which the nip width that arises during operation in the region of the narrowest nip is greater than the granular size of the grinding stock pressed there. Moreover, the invention relates to a grinding apparatus for carrying out the method.
In the pressure comminution of granular material in the nip of a high-pressure roll mill or, respectively, roll press, the bulk material supplied to the nip is seized by the rolls, which rotate in opposite directions, and is drawn into the nip by friction. The individual particles of the bulk material drawn in are thereby mutually crushed, grain against grain, in a bed of material, i.e., in a material grist pressed together between the two roll surfaces under the application of a high pressure. This arrangement is described as interparticle crushing and is disclosed in DE-C 27 08 053 (which disclosure is incorporated herein by reference). This pressure treatment or, respectively, material pressing produces material agglomerates made of comminuted bulk material which can subsequently be broken up or, respectively, disintegrated with a relatively low mechanical outlay, whereby the overall low specific energy requirement of the interparticle crushing results.
The energy savings is greatest in pure interparticle crushing. The granular size of the feed material must then be smaller than the nip width of the high-pressure roll press. Given coarse feed material, at least one coarse crushing machine is thus required, which comminutes the overall feed material to a granular size that is smaller than the nip width of the interparticle crushing roll mill. Thus, it is proposed for example in EP-B-0 278 858, FIG. 1, to connect a breaking mill provided with roll profiles upstream from a interparticle crushing roll mill, in which breaking mill the entire feed material is coarse-crushed. The upstream breaking mill can likewise be an interparticle crushing roll mill, in whose nip the overall bulk material supplied, i.e. also particles whose granular size is already sufficiently small, are comminuted, whereby the specific energy requirement of the overall milling process is however increased.
On the other hand, it is known to make the highly stressed roll surfaces of the interparticle crushing high-pressure roll mills or, respectively, roll presses more resistant to wear by means of what is known as a grid armoring, by attaching a multiplicity of pre-manufactured profile rails or pin-type nap bolts to the roll surface (EP-B-0 443 195 (U.S. Pat. No. 5,203,513), e.g. FIG. 4). The nap bolts thereby protrude outward from the roll surface with such a height, and are arranged with such a spacing from one another, that during operation of the roll press, the intermediate spaces or, respectively, pockets on the roll surface between the nap bolts remain filled with the pressed-together fine granular material, which forms an autogenous wear protection for the roll surfaces.
It is known that in the interparticle crushing, in particular of moist ores (moistness between approximately 2 and 10%), as they occur in practice, the adhesion or, respectively, the stability of the autogenous wear protection layer on the roll surface of the roll press provided with the grid armoring is not always ensured. It has thus already been proposed to use a binding agent to first attach a highly wear-resistant adhesive layer made of fine granular foreign material, such as e.g. silicon carbide, to the roll surfaces provided with the nap bolts, whereby during roll press operation the autogenous wear protection layer, e.g. the layer made of interparticle-crushed highly compressed ore agglomerates, can remain in adhesion to the rough surface of this adhesive layer. However, the service life of the autogenous wear protection layer, as well as of the radial inner adhesive layer, is endangered when an oversized particle or, respectively, scatter particle makes its way into the nip of the interparticle crushing high-pressure roll press, which, given an individual particle crushing (instead of interparticle crushing) in the nip of the roll press can lead to premature destruction of the autogenous wear protective layer, as well as of the adhesive layer located thereunder. This is also the case if, in an interparticle crushing roll press in whose narrowest nip region the interparticle crushing is carried out, an individual particle crushing were constantly carried out superposed thereon, in the upper additional nip region (DE-C 33 02 176 (U.S. Pat. No. 4,703,897), FIG. 1).
It would be disadvantageous and uneconomical to separate out existing rough particles/oversized particles/scattered particles from a moist granular mineral stream by means of a sieve classification, especially because the separated rough particles/oversized particles/scattered particles would then have to be ground separately.